1. Field of the Invention
The present invention relates to transmitters and receivers which use resource sharing and coding to provide increased capacity and sufficient link margin and, more particularly, to transmitters and receivers for use in time division multiple access (TDMA) communication systems such as, for example, a digital satellite system wherein the resource sharing concept is generalized by fully exploiting the available clear-air carrier-to-noise ratio of the satellite link to achieve very high transmission capacity while maintaining low rain outage. During clear-air conditions, redundancy codes, as for example a convolutional code, and a large channel signaling alphabet are employed to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, the signaling alphabet may be reduced and enough time slots, or portions of a time slot, are borrowed from a resource sharing reserved pool to maintain the data rate at the fade site.
2. Description of the Prior Art
The current trend in communication satellites appears to be increasingly toward the use of the 12/14 GHz and higher frequency bands and the use of digital modulation formats with Time Division Multiple Access (TDMA) techniques. The former provides freedom from existing 4/6 GHz terrestrial interference and also provides higher antenna gain and narrower beams for a given size aperture, while digital transmission in conjunction with TDMA provides for more efficient utilization of the available satellite system resources.
A major drawback associated with 12/14 GHz systems is the signal attenuation associated with rainfall. In general, attenuation at these frequencies is an increasing function of rain rate, with the result that, for example, over a large portion of the United States, significant power margin must be provided to prevent excessive outage due to rain fades.
A typical prior art technique for overcoming rain fades is disclosed in an article "The Future of Commercial Satellite Telecommunications" by W. White et al in Datamation, July 1978 at pp. 94-102 which discloses at pp. 98-99 that it may be possible to overcome rain attenuation in satellite systems by transmitting the same burst several times. The ground station in the momentary rain zone can add the multiple signals for the same burst together to reconstruct the original signal.
Other standard techniques which might be employed to provide rain margin include (1) increasing the radiated power of the satellite and earth stations, (2) improving the noise figure of the receivers, (3) installing larger ground station antennas, and (4) providing site diversity. Unfortunately, these techniques (1)-(4) are costly in that permanently dedicated system resources are used only infrequently, i.e., when it rains. Therefore, the system has been tremendously overdesigned for the clear air conditions which might exist more than 99.9 percent of the time at any particular ground location if, for example, 15 or 20 dB rain margin is required to achieve the desired rain outage.
Techniques for increasing the rain margin of a satellite communication system without the requirement of additional system resources are described in U.S. Pat. No. 4,309,764 issued Jan. 5, 1982 for A. Acampora and U.S. Pat. No. 4,301,533 issued Nov. 17, 1981 for A. Acampora et al, and assigned to the same assignee as the present invention. The invention of each of these patents uses regular uncoded symbol-by-symbol transmissions between ground stations via a satellite when no fade condition exists at either the transmitter or receiver, but spare concurrent TDMA time slots in each frame sequence are obtained from a pool or by rearrangement of spare time slot assignments for use in communications with ground stations experiencing rain attenuation events which exceed a predetermined power margin.
In U.S. Pat. No. 4,309,764 during rain attenuation events, additional power margin is provided by each transmitter using burst extension and coding techniques. Each transmitter, therefore, must include means which can be switched to provide the appropriate nonfade-uncoded or fade-encoded preamble and message information to enable transmission to (a) nonfaded receivers, (b) faded receivers or (c) transmission to a nonfaded receiver where the transmitter experiences a fade condition and increased power transmission is not available. At each receiver which can experience a fade or receive encoded information from a faded transmitter not capable of increasing transmission power, each receiver includes means which can be switched to receive and process unity or greater extended field frame synchronization signals, carrier and clock signals, start of message signals and other encoded preamble and data information destined for the receiver to overcome the fade condition.
In U.S. Pat. No. 4,301,533 arrangements are disclosed wherein additional down-link margin is provided to a station experiencing a rain fade condition by the use of concurrent pool or rearranged spare time slots associated with each down-link beam frame format to permit the power normally transmitted in multiple concurrent beams from the satellite to be applied in effectively one or more beams directed at the ground station or stations experiencing the fade condition.
The problem remaining in the prior art is to provide a technique which provides sufficient power margin in communications between two remote stations during either clear air or rain fade periods without the requirement of additional system resources and which maximize the data throughput and minimize outage time due to insufficient power margin.